The response for REL regarding subwoofers

I think there is a limit to how much of a spread changing the gain on the subwoofer amp can manage. 
 

The average dynamic speaker is sitting between 87-95 DB efficiency. The Klipschorn is 105! 

At half a watt the KH is putting out 102DBs! The amount of gain needed for an amplifier to hit that output is very very very small. It gets worse as you get into more normal listening levels.

Think of how little gain would be needed to play those Khorns @ 85DB. 
 

The Rels, being probably more average in efficiency, would need a significantly higher amount of gain to match the output. 
 

Much higher than the gain difference on the subwoofer can accommodate.

Great explanation!

You could probably accomplish what you wanted with a pair of No.25 subs from REL, depending on room size and how loud you like to playback your music. 

That makes sense if you’re using the speaker level input. There might be other options, though you’d need to be careful and run the numbers.

With the speakers being so efficient, depending on your listening levels, the speaker level voltage could be within the limits of the line level inputs of the sub. There might be a way to modify the sub’s high level inputs for less attenuation as well.

You could run a sub out from your preamp at line level, but attenuate the signal going into the power amp and to the speakers. This depends on your amp input impedance. A passive pre amp would give additional adjustment. I think some attenuators can have roll off if the output impedance of the pre is too low. 

I’m sure there are variations of these, but it would depend on your specific setup and equipment.